Metal silicides have attracted increasing attention recently as possible interconnect material for semiconductor integrated circuits. In particular, bilayer composite films of metal silicide and polycrystalline silicon have been found to provide low resistance gate and interconnect layers for MOS circuits and to be otherwise compatible with current silicon gate technology.
Tungsten disilicide has received particular attention as a candidate for such applications because its electrical resistivity is among the lowest of the metal silicides. In addition, tungsten disilicide grows a passivating silicon oxide layer if a silicon source is available and it is not attacked by HF solutions. Films of tungsten disilicide have been formed by sintering films of tungsten metal deposited on silicon by evaporation, sputtering, or chemical vapor deposition (CVD). They have also been formed by sintering mixed films of tungsten and silicon formed by co-evaporation, co-sputtering, or sputtering from a tungsten disilicide target.
In the fabrication of semiconductor integrated circuits, CVD methods are often preferred to physical deposition processes because CVD metal silicide films usually show more even step coverage than sputtered or evaporated film and the costs can be lower. In addition, CVD methods are production oriented and can result in purer films.
A basic problem, however, in depositing metal silicides by CVD methods is that it is sometimes difficult to deposit the metal component at the temperatures required for safe semiconductor device fabrication. For example, typical deposition temperatures for tantalum or titanium are over 1000.degree. C., a temperature at which the junctions of a semi-conductor integrated circuit become damaged. A convenient temperature for deposition of metal silicides would be about 600.degree.-650.degree. C.